We had previously developed a radioreceptor assay to measure insulin and growth hormone using IM-9 cultured lymphocytes as the receptor and labeled polypeptide hormones as the ligand. We have found that human growth hormone and cycloheximide alters the concentration of the growth hormone receptor on these cells. A decrease in receptor binding is dependent upon the concentration of the agents present as well as the duration of exposure of the agents to the cells. The decrease in receptor concentration is a reversible process and protein synthesis seems to be required for the restoration of receptors. The in vitro studies allow one to study the effects of agents upon the hormone specific receptors.